Metathesis unit pretreatment process with formation of octene

ABSTRACT

Disclosed is a process for integrating a butene dimerization process with a metathesis process to remove isobutene from the feed stream to the metathesis reactor. The isobutene is preferentially dimerized in the dimerization process to leave n-butenes for metathesis with ethylene. An upstream selective hydrogenation process also isomerizes 1-butenes to 2-butenes which is the preferred butene reagent in the metathesis process. A common fractionator column for the dimerization and hydrogenation processes is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of copending application Ser. No.12/263,879 filed Nov. 3, 2008, which is a Division of application Ser.No. 11/283,353 filed Nov. 18, 2005, now U.S. Pat. No. 7,459,593,thecontents of which are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The invention is a process for converting an olefinic C₄ stream intopropylene and octene. Specifically, the invention relates to anintegrated dimerization and metathesis process.

BACKGROUND OF THE INVENTION

Steam cracking processes are used to produce ethylene and propylene asbasic petrochemicals. Steam cracking processes also produce olefinic C₄hydrocarbons such as butene-1, butene-2, isobutene, butynes andbutadiene. The demand for these compounds as petrochemical feedstocks isless than the quantities produced, and hence various approaches havebeen suggested for converting surplus C₄ olefins into more valuableproducts.

One process for conversion of olefinic C₄ streams is to carry outmetathesis with ethylene to form propylene. This is a particularlyattractive process, as it gives the steam cracker operator theflexibility to adjust the ratio of ethylene to propylene that isproduced in the complex. The preferred feed for a metathesis plant is astream rich in butene-2. Isobutene, butynes and butadiene are notdesirable feeds to the metathesis process as these compounds causedeactivation of the metathesis catalyst and/or higher coke make in themetathesis process. Butene-1 is not a desirable feed to the metathesisprocess as it will react with butene-2 to produce pentenes and withitself to produce hexenes. These compounds are low-value byproducts.Some form of pretreatment is, therefore, normally applied to theolefinic C₄ stream to produce a stream rich in butene-2 that can be usedas a metathesis feed.

The current state-of-the-art process for removal of isobutene from ametathesis feed is to react the isobutene to form methyl tert-butylether (MTBE), ethyl tert-butyl ether (ETBE) or tert-butyl alcohol (TBA).These processes can be carried out at greater than 99.9% conversion ofisobutene with negligible conversion of normal butenes and, hence,permits high propylene yields in the metathesis process. The phase outof MTBE as a gasoline additive has reduced the economic advantage ofthis conversion route and led to a need for new processes that convertisobutene into more valuable products.

An alternative process for the removal of isobutene from a metathesisplant feed is to react away the isobutene in a polymerization process.If the entire C₄ stream is subjected to polymerization conditions thensubstantial conversion of normal butenes also occurs, which reduces theoverall yield of propylene. Furthermore, separation of isobutene fromnormal butenes is difficult as isobutene boils very close to butene-1.

As described below, various methods have been proposed for preparing ametathesis feed from an olefinic C₄ stream. There remains, however, aneed for improving the value of the products produced from suchprocesses and for reducing the capital and operating costs of suchprocesses.

RELATED ART

It has been recognized in the art that an olefinic C₄ stream mustundergo pretreatment prior to metathesis. U.S. Pat. No. 6,075,173 issuedto J. Chodorge et al. teaches removal of dienes and butynes using aselective hydrogenation process in which isomerization of butene-1 tobutene-2 also occurs. The product of the selective hydrogenation processis distilled to give an overhead stream rich in isobutene and butene-1and a bottoms stream rich in butene-2 and butane. It is taught that theoverhead stream can undergo isobutene polymerization and the bottomsstream can be used as a metathesis feed.

U.S. Pat. No. 6,207,115 issued to J. Chodorge et al. discloses a processin which separation of isobutene from butene-2 prior to polymerizationis optional, and the entire olefinic C₄ stream can be subjected topolymerization conditions to react away isobutene. The patent alsoteaches that the olefinic C₄ stream may be augmented with the productsfrom metathesis of an olefinic C₅ stream.

U.S. Pat. No. 6,538,168 issued to P. Schwab et al. discloses a processin which dienes and butynes are removed by extraction or selectivehydrogenation, followed by removal of isobutene by polymerization,oligomerization or reaction with an alcohol to form an ether. Butene-1and butene-2 are not separated prior to metathesis, and the resultingproduct contains a high fraction of C₅ and C₆ olefins as well aspropylene.

U.S. Pat. No. 6,646,172 issued to P. Schwab et al. describes a processin which isobutene and butene-1 are not separated from butene-2 prior tometathesis. The resulting C₅ olefin products are subjected to a furthermetathesis step with ethylene to give additional propylene andregenerate the butenes.

U.S. Pat. No. 6,686,510 issued to D. Commereuc et al. discloses aprocess for pretreating a metathesis feed and forming a high purityisobutene product. The olefinic C₄ stream is selectively hydrogenated toremove dienes and butynes and then distilled in a reaction distillationcolumn that incorporates a catalyst for hydroisomerization of butene-1to butene 2.

U.S. Pat. No. 6,872,862 issued to R. Bridges et al. teaches treating theC₄ stream by selective hydrogenation, followed by distillation toseparate the butene stream into an isobutene-rich overhead product and abutene-2-rich bottom product that is sent to a metathesis reactor. Thepatent teaches sending the isobutene-rich overhead stream to a skeletalisomerization process, in which isobutene is converted to normal butenesthat can be recycled to the process feed, hence increasing the overallyield of propylene.

It is also known to those skilled in the art that dimerization ofisobutene may be used to form an olefinic stream rich in octene that canbe blended into motor gasoline. U.S. Pat. No. 4,244,806 issued to J. LePage et al. discloses a process for forming a stream from the productsof isobutene polymerization that is suitable for blending into motorgasoline. The patent notes that a polymerization reactor that is rununder selective conditions for dimerization and trimerization willproduce a reactor product that contains a significant amount ofunreacted butenes. The patent teaches that this limitation can beovercome by subjecting the product of the polymerization reactor todistillation to recover as overhead product a stream comprisingunreacted butenes and butanes that can be sent to an alkylation process.

U.S. Pat. No. 4,393,259 issued to D. Ward et al. describes a process forproducing gasoline from propane or butane, in which an alkane stream isdehydrogenated to form an alkene-rich stream that is then subjected tocatalytic condensation in the presence of a solid phosphoric acid (SPA)catalyst at a pressure in the range from 15 to 1200 psig and atemperature in the range from 120° to 260° C., to form dimers andtrimers. Unconverted C₃ and C₄ compounds are separated from the dimersand trimers by distillation and recycled to the dehydrogenation zone.

U.S. Pat. No. 4,469,911 issued to H. Manning discloses a process forisobutene oligomerization in the presence of a fixed bed cation exchangeresin at a temperature in the range from 30° to 60° C. and a liquidhourly space velocity in the range from 2.5 to 12 hr⁻¹.

U.S. Pat. No. 5,895,830 issued to L. Stine et al. describes animprovement in the dimer selectivity of a butene oligomerization processthat uses SPA catalyst, caused by diluting the butene feed with a heavysaturate stream comprising paraffins having a carbon number of at least8.

U.S. Pat. No. 5,877,372 issued to T. Evans et al. discloses dimerizationof isobutene in the presence of isooctane diluent and tert-butylalcohol, over a sulfonic acid type ion exchange resin such as AmberlystA-15, Dowex 50 or the like, at temperatures in the range 10° to 200° C.and pressures in the range of 50 to 500 psig. It is suggested thattert-butyl alcohol improves the selectivity of dimer formation andreduces the formation of trimer and higher oligomers. The amount ofselectivity modifier that is suggested is at least 1 wt-% and preferably5 to 15 wt-%.

U.S. Pat. No. 6,689,927 issued to R. Frame et al. describes a lowtemperature butene oligomerization process having improved selectivityfor dimerization and improved selectivity for the preferred2,4,4-trimethylpentene isomer, caused by carrying out oligomerization inthe presence of an SPA catalyst at a temperature below 112° C. in thepresence of a saturated hydrocarbon diluent having a carbon number of atleast 6.

SUMMARY OF THE INVENTION

It has been discovered that a significant improvement in the overallperformance of the metathesis feed pretreatment complex can be obtainedif the olefinic C₄ stream is separated to give an isobutene-rich streamthat is sent to a dimerization process to produce an octene product thatcan be used as an olefinic motor gasoline alkylate fuel.

It has further been discovered that the capital and operating costs ofsuch a process can be substantially reduced by combining thedistillation operation of the dimerization process with the distillationoperation of the metathesis pretreatment process in a singledistillation column.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a simplified process flow diagram showing a process fortreatment of an olefinic C₄ stream to produce propylene and octene.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention is illustrated in the drawing, which isintended only to describe one embodiment of the invention and is notintended to limit either its application or scope. Referring now to thedrawing, a C₄ stream rich in olefinic compounds is passed through aprocess line 1 into a selective hydrogenation process 2. A second feedstream rich in hydrogen is introduced into the selective hydrogenationprocess 2 through process line 3. The feed to the subject process can beany stream that is rich in C₄ olefinic compounds, for example a C₄stream from a steam cracking process, from a catalytic cracking process,from a metathesis process, from a butane dehydrogenation process, froman ethylene dimerization process or from another refinery process thatproduces C₄ olefins. The feed may comprise butene-1, butene-2,isobutene, butadiene, butynes, isobutane, normal butane and smallamounts of other hydrocarbons such as C₂ compounds, C₃ compounds, C₅compounds and C₆ compounds. It is well known to those skilled in the artthat butynes and dienes easily undergo polymerization reactions thatlead to formation of coke and reduce the effectiveness of catalysts usedin downstream processes. The selective hydrogenation process istherefore operated in such a manner to partially hydrogenate butynes andbutadiene to form mainly butene-1 and butene-2, without substantiallyconverting butenes into butanes. Suitable conditions for operation of aselective hydrogenation process are well known to those skilled in theart and are described, for example, in U.S. Pat. No. 6,166,279 and U.S.Pat. No. 6,075,173. Such conditions include passing the C₄ mixture inthe liquid phase in the presence of hydrogen at molar ratio 0.5 to 5moles hydrogen per mole of diolefin over a catalyst comprising at leastone metal selected from the group formed by nickel, palladium andplatinum, deposited on a support such as aluminum oxide, at atemperature of 200 to 200° C., a pressure of 689 to 3447 kPa(g) (100 to500 psig), and a space velocity of 0.5 to 10 hr⁻¹. Two or more reactionzones may be used, and each reaction zone may employ a recycle ofreactor effluent to the reactor inlet with a ratio of recycle to fresholefinic feed stream ranging from 0 to 20. The residual butadienecontent of such a process can be in the range 1 to 100 wppm, dependingon the severity of the operation. The selective hydrogenation processalso causes isomerization of butene-1 to butene-2, as described in U.S.Pat. No. 6,166,279. In the process of the invention, such isomerizationis desirable, as it increases the overall yield of propylene from themetathesis process. The selective hydrogenation process is preferablycarried out under conditions such that the ratio of butene-2 to butene-1in the product is greater than 10, and more preferably greater than 12.The selective hydrogenation process may consist of a first reaction zoneprimarily for butadiene hydrogenation prior to a second reaction zoneprimarily for butene isomerization.

A partially hydrogenated effluent stream is withdrawn from the selectivehydrogenation process 2 through process line 4 and is fed to adistillation column 5. In distillation column 5, butene-1 and compoundsboiling at lower temperatures than butene-1 are removed as an overheadfraction through process line 6. The overhead fraction is sent to apartial condenser 7, where C₄ compounds are condensed to form anisobutene-rich liquid stream that is withdrawn through process line 8,while uncondensed hydrogen and light hydrocarbon compounds are withdrawnas a vapor product through process line 9. A portion of theisobutene-rich liquid stream is sent through process line 10 todistillation column 5 to serve as a reflux for the distillation column.

A second portion of the isobutene-rich liquid stream is sent throughprocess line 11 and subsequently mixed with a diluent that entersthrough process line 12, and/or a selectivity modifier that entersthrough process line 13, to form a dimerization reactor feed that isthen fed to a dimerization reactor 14. In accordance with the process ofthe invention, the dimerization reactor should be operated underconditions that favor the formation of octene, while substantiallyrestricting the formation of dodecene and higher oligomers of isobutene.Octene is defined as an olefin consisting of eight carbon atoms, not allnecessarily in a straight chain with octane defined as the saturatedform of octene. Conditions for the operation of a dimerization processinclude passing the isobutene-rich liquid over a catalyst such as SPA ora sulfonic acid ion exchange resin such as Amberlyst A-15, A-35, A-16,A-36, Dowex 50 or the like. Several means can be used to restrict theformation of dodecene and higher oligomers of butene. These includeaddition of a diluent to the dimerization reactor feed stream, recycleof a portion of the dimerization reactor effluent to the dimerizationreactor feed stream and addition of a selectivity modifier to thedimerization reactor feed stream. Suitable diluents include paraffinichydrocarbon compounds selected from the group comprising propane,isobutane, normal butane, isopentane, normal pentane, octane andisooctane. Higher paraffins may be a preferred diluent if SPA is used asthe oligomerization catalyst. In a preferred embodiment of the presentinvention, the diluent is a normal butane purge stream recovered from ametathesis process. Use of selectivity modifiers is preferred when theoligomerization catalyst is a resin catalyst. Suitable selectivitymodifiers include oxygenated compounds selected from the groupcomprising water, tert-butyl alcohol and sec-butyl alcohol. Otheroxygenates may also be present. Typically, the selectivity modifiershould be about 0.1 to about 3.0 wt-% and preferably about 0.5 to about2.5 wt-% of the fresh feed when operating with a resin catalyst.

As is known to those skilled in the art, the preferred operatingconditions when an SPA catalyst is used differ from those when an ionexchange resin catalyst is used. For example, when an SPA catalyst isused, the presence of a selectivity modifier, oxygenate compound is notrequired and a recycle of saturated octane product is preferred.Preferred relative proportions of isobutene in the feed to the reactorfor operation of the dimerization process are at least 5 wt-% andpreferably at least 20 wt-%. Isobutene preferably will comprise at least33 wt-% of the total butenes fed to the reactor. The preferred ratio ofparaffinic diluent to olefins in the feed to the reactor will be about1.5 to about 3.5 for operation with SPA catalyst. The preferred ratio ofparaffinic diluent to olefins in the feed to the reactor will be about0.5 to about 3.0 for operation of the dimerization process with a ionexchange resin catalyst. Preferably, the diluent flow rate is adjustedto maintain the isobutene concentration in the feed to the reactor to nomore than 55 wt-% for operation with either SPA or resin catalyst.Preferred temperatures for operation with an SPA catalyst are in therange 40° to 260° C., and more typically in the range 75° to 230° C.,while preferred temperatures for operation with an ion-exchange resincatalyst are in the temperature range 0° to 200° C., and more typicallyin the range 40° to 150° C. Preferred pressures for operation with anSPA catalyst are in the range 689 to 8274 kPa(g) (100 to 1200 psig), andmore typically in the range 1379 to 6895 kPa(g) (200 to 1000 psig),while preferred pressures for operation with an ionic resin catalyst arein the range 345 to 3447 kPa(g) (50 to 500 psig), and more typically inthe range 1379 to 2413 kPa(g) (200 to 350 psig). A preferred spacevelocity range for operation with SPA catalyst is about 0.5 to about 5hr⁻¹ and for operation with an ion-exchange resin catalyst is 0.3 to 20hr⁻¹ depending on the properties of the dimerization reactor feed suchas olefin content and type.

A dimerization reactor product is withdrawn from dimerization reactor 14through process line 15. A portion of the dimerization reactor productmay be recycled to the dimerization reactor feed through process line16. A second portion of the dimerization reactor product is passedthrough process line 17 to a flash drum 18, in which a C₄-rich vaporstream and an octene-rich liquid stream are formed. The C₄-rich vaporstream leaves flash drum 18 in process line 19 for further processing. Aportion of vapor stream in line 19 may be recycled by line 60 to thedimerization reactor 14 after condensing and compression while theremaining stream is processed through line 61. The octene-rich liquidstream is sent through process line 20 to distillation column 5.Returning the octene-rich liquid to distillation column 5 permits therecovery of unconverted butene compounds that may be present in thisstream, while at the same time obviating the use of an additionaldistillation column within the dimerization section of the process. Theoctene-rich liquid is preferably fed to distillation column 5 at aposition in the distillation column that is below the position at whichthe partially hydrogenated effluent stream is fed to the distillationcolumn.

In one embodiment of the invention, not illustrated in the drawing, theC₄-rich vapor stream that leaves flash drum 18 in process line 19 or 61,if the vapor recycle is utilized, is mixed with a second diluent streamand a second selectivity modifier stream and is sent to a seconddimerization reactor to form additional octene. A portion of the seconddimerization reactor product may be recycled to the second dimerizationreactor feed. A second portion of the second dimerization reactorproduct is sent to a second flash drum, in which a second C₄-rich vaporstream and a second octene-rich liquid stream are formed. The secondC₄-rich vapor stream can be sent to an additional dimerization reactor,or else can be sent for recovery of butanes. The second octene-richliquid is combined with the first octene-rich liquid in process line 20and is returned to distillation column 5.

A bottoms product is withdrawn from distillation column 5 throughprocess line 21, comprising substantially all of the octene that is fedinto distillation column 5 through process line 20. The recovery ofoctene in the bottoms product is at least 98%, preferably at least 99%and more preferably at least 99.9%. The octene product may be used as anolefinic motor gasoline blending component. The bottoms product in line21 may be processed through an alcohol recovery process, not shown,which comprises a water wash to remove the alcohol and water from thehydrocarbon product and a distillation column to separate the water fromthe alcohol. Other extraction or adsorption processes may be used toseparate alcohol and water from the hydrocarbon. The alcohol may then berecycled to rejoin the selectivity modifier in line 13. In oneembodiment of the process, the bottoms product in line 21 with orwithout alcohol may be sent to a hydrogenation process unit 22, in whichoxygenated hydrocarbon compounds and residual olefins are converted byhydrogenation to form a hydrogenated octane-rich product that issuitable for blending into motor gasoline and leaves the process inprocess line 23. Suitable conditions for operation of such a process aredescribed in U.S. Pat. No. 6,548,721, and include contacting the bottomsproduct in the presence of hydrogen with a saturation catalystcomprising a metal from the top row of Group VII of the Periodic Tableof the Elements and a metal from Group VI-B of the Periodic Table of theElements, at a temperature of at least 200° C., a pressure in the rangefrom 1724 to 4482 kPa(g) (250 to 650 psig) and a liquid hourly spacevelocity (LHSV) in the range from 1.5 to 15 hr⁻¹. Preferably, thesaturation catalyst includes at least 5 to 15 wt-% molybdenum and atleast 5.5 wt-% sulfur.

A mid-cut product is removed from distillation column 5 through processline 24 and is sent to a metathesis reactor guard bed 25 in whichcontaminants such as sulfur, oxygenated hydrocarbon compounds andnitrogen are removed by adsorption to form a contaminant-free C₄metathesis process feed which exits in line 26. A supplementary C₄+stream may be introduced to line 26 exiting from the guard bed 25. Anethylene-rich feed stream may also be introduced into line 26 throughprocess line 28. Process line 28 preferably introduces ethylene-richfeed to process line 26 to premix reactants prior to entering reactor27, but independent entry of line 26 into reactor 27 is contemplated.The metathesis process feed is sent through process line 26 to ametathesis reactor 27 preferably after being heated. In the metathesisreactor 27, ethylene and 2-butene produce propylene. A metathesisreactor product containing ethylene, propylene and butenes is removedfrom metathesis reactor 27 through process line 29.

The mid-cut product is withdrawn from distillation column 5 at a pointintermediate between the points where the overhead and the bottomfractions are taken. Preferably, the mid-cut product is withdrawn fromdistillation column 5 at a point intermediate between the points wherethe partially hydrogenated effluent stream and the octene-rich liquidare fed. The point at which the mid-cut product is withdrawn is chosenso as to produce a mid-cut stream that is rich in butene-2, whilemaintaining a concentration of isobutene in the mid-cut that ispreferably less than 1 wt-%, while also maintaining a recovery of octenein the bottoms product that is preferably greater than 98%. In oneembodiment of the invention, distillation column 5 may contain adividing wall or a partition wall, extending upwards from a point belowthe location at which the octene-rich liquid is fed to a point above thelocation at which the mid-cut is withdrawn, so as to prevent isobutenethat enters with the octene-rich liquid from accumulating in themid-cut.

Conditions for the operation of metathesis reactor 27 vary widely, buttypically include contacting a mixture of ethylene and butene with ametathesis catalyst comprising at least one of halides, oxides and/orcarbonyls of at least one of molybdenum, tungsten, rhenium and ormagnesium on a support such as silica, alumina or silica-alumina at atemperature of from about 38° to 427° C. (100° to 800° F.), a pressurefrom about 1379 to 4137 kPa(g) (200 to 600 psig) and a weight hourlyspace velocity of about 1.0 to 100 hr⁻¹. Typically the metathesiscatalyst comprises magnesium oxide and tungsten oxide on silica orrhenium heptoxide deposited on a gamma alumina with a rhenium content(expressed as rhenium metal) in the range of 1 to 15 wt-%.

In an embodiment, the metathesis product may be fed to a deethanizercolumn 40 from which an overhead product comprising ethylene and lightergases may be withdrawn, cooled and separated in separator 42. A lightvapor stream may be purged from the separator 42 in line 41. If thelight vapor stream is rich in hydrogen, it may be recycled to line 3with an appropriate purge. Part of the liquid stream is refluxed to thecolumn 40 and the remainder rich in ethylene is recycled in line 44 tosupplement ethylene feed in line 28. A bottom stream is removed and fedto a depropanizer column 50 in line 46 after a portion is reboiled andreturned to the deethanizer column 40 in line 47. An overhead streamcomprising propylene may be withdrawn and cooled to provide propyleneproduct in line 51 while a portion of the overhead is refluxed to thedepropanizer column 50. A bottoms product of C₄+ material rich in2-butene may be withdrawn from the depropanizer in line 54 and recycledto process line 26 which feeds 2-butene to the metathesis processdownstream of the guard bed 25. A portion of the bottoms stream in line56 may be split into two streams including a reboiler stream that isreboiled and fed to the depropanizer column 50. The other stream fromthe split in line 58 may serve as a purge from the metathesis processand may be used to supplement diluent stream in line 12 still with anappropriate purge if necessary.

1. A process for segregation of olefinic C₄ compounds from at least onefeed stream containing butene-2 and at least one feed stream containingoctene comprising: a) introducing at least one feed stream comprisingisobutene, butene-1 and butene-2 into a distillation column; b)introducing at least one feed stream comprising octene, isobutene andisobutane into the distillation column; c) removing an overhead productcomprising hydrogen, isobutene and butene-1 from the distillationcolumn; d) removing a bottoms product comprising octene from thedistillation column; and e) removing at least one mid-cut productcomprising butene-2 from the distillation column.
 2. The processaccording to claim 1, wherein the feed streams comprising isobutene,butene-1 and butene-2 are fed to the distillation column at positionsabove the positions where the feed streams comprising octene, isobuteneand isobutane are fed to the distillation column.
 3. The processaccording to claim 1, wherein the mid-cut product is removed from alocation below the lowest point at which a stream comprising butene-2 isfed and above the highest point at which a stream comprising octene isfed.
 4. The process according to claim 1, in which a portion of theoverhead product of step (c) is sent to a dimerization reactor.
 5. Theprocess according to claim 1, in which the mid-cut product of step (e)is combined with a stream comprising ethylene and is sent to ametathesis reactor.